The present application relates to a reflective display device such as a reflective liquid crystal display device (LCD). It particularly relates to a reflective display device having a light source to provide illumination, for example in poor light condition. It also relates to a light source, particularly but not necessarily for use with a display device.
Reflective LCDs are known in the art. For example, the xe2x80x9csuper-mobilexe2x80x9d high reflectivity LCD produced by Sharp Kabushiki Kaisha essentially comprises a polarizer, waveplates, and a liquid crystal layer disposed in front of a reflective layer. The reflective layer has a metallized non-symmetric surface relief structure, which substantially preserves the polarization of light incident on the reflective layer so as to maximize the contrast of the display. The reflective layer is a xe2x80x9chigh-gainxe2x80x9d reflective diffuser using a non-Lambertian surface relief structure.
Such a reflective LCD can use ambient light, so that the power consumption of the LCD is reduced compared to an LCD that comprises a light source provided behind the liquid crystal layer. This is important for portable display equipment such as a personal digital assistants (PDA), camcorders, portable computing equipment, and digital cameras.
Such a conventional reflective LCD does have a number of disadvantages. Clearly, the display will work poorly in dimly lit environments. Moreover, the color balance of the display will be influenced by the color spectrum of the ambient light. Thus, in sunlight, the display device may work well, but in fluorescent or particularly tungsten lighting, the color balance will be degraded. If the LCD is provided with color filters, these may also affect the color balance of the display.
A further disadvantage is that a high-gain reflective layer will work best when the light incident on the LCD is highly collimated overhead lighting incident on the screen of the LCD at a defined angle. Thus, the display position may have to be fixed by the user of the LCD to obtain the best performance, and this is undesirable. It can thus be seen that it would be advantageous to provide a reflective LCD with an auxiliary light source, to increase the quality of the displayed image and also to allow the LCD to operate in conditions of low ambient light.
As described above, a conventional reflective LCD has a reflective layer that is a high-gain reflective diffuser. If a light source is placed close to the surface of the LCD, a number of effects will be produced. First, the gain of the reflective layer will mean that the observer will see illumination structure across the surface of the LCD. This illumination structure will depend on the profile of illumination by the light source, and on the gain of the reflective layer.
Secondly, some light from the light source will be specularly reflected by the front surface of the LCD and from layers within the LCD. If the light source and a user are positioned such that light from the light source is specularly reflected towards the user, this will reduce the contrast of the display and will also be uncomfortable for the user. In order to prevent the user from seeing the specular reflection of the light source, the light source may have to be displaced laterally, i.e., offset, from the display surface. Displacing the light source in this way does, however, introduces a further problem. The LCD may well provide the highest display contrast for light within well-defined input and output cones. If the light source is displaced as described above, the input light may well no longer lie within the preferred input cone to the LCD and this will reduce the imaging efficiency.
FIGS. 1A and 1B show a prior art reflective LCD. Specifically, FIG. 1A is a cross-sectional view of a prior art reflection LCD device with a holographic element for brightness enhancement in front illumination, and FIG. 1B illustrates the incident and reflective light for the device of FIG. 1A.
The illustrated device is based on a glass substrate 1 on which are disposed, in sequence, a reflective layer (internal mirror) 2, a liquid crystal layer 3, color filters 4, an upper glass substrate 5, a polarizer 6 and a transmission hologram 7. Electrodes to drive the liquid crystal layer 3 are provided on the glass substrates 1 and 5.
The transmission hologram 7 is provided to enhance the brightness of the display. The LCD is illuminated by a distant light source, positioned at 34xe2x80x3 offset from the normal axis of the display as shown in FIG. 1B. Light that is specularly reflected from the front surface of the display, or from internal elements of the LCD, is reflected at an angle of 34xe2x80x3 to the normal to the display.
The transmission hologram 7 deflects light that is incident upon it. Thus, some of the light that passes through the liquid crystal layer 3 and is reflected by the reflective layer 2 is reflected back towards an observer much closer to the axis of the display than the specularly reflected light. As shown in FIG. 1B, the light from the display is reflected back at an angle of 14xe2x80x3 to the normal axis, whereas the specularly reflected light (xe2x80x9cglarexe2x80x9d) is reflected at 34xe2x80x3 to the normal axis. The brightness of the LCD is thus increased, since the display light is angularly separated from the glare.
The prior art display shown in FIG. 1A requires to be illuminated with a distant light source that appears collimated at the liquid crystal panel, and it would not function efficiently if the light source were close to the LCD.
U.S. Pat No. 5,663,816 discloses a transmissive LCD in which a reflective holographic directional diffuser is provided at the back side of the LCD. The holographic diffuser is transparent for illumination by a back light, but directionally reflects light when the LCD is illuminated by an external overhead source. The reflection is conical about the axis perpendicular to the reflection side of the LCD.
FIG. 2 shows a further prior art reflective LCD with a holographic light control film. This prior art LCD is disclosed in T. Hotta et al., SPIE Proc., Vol.329, Practical Holography XII, pages 190-195, 1998.
The LCD of FIG. 2 comprises a liquid crystal layer 15 which is disposed between two glass substrates 13, 17 provided with electrode layers 14, 16 for applying voltages to the liquid crystal layer 15. A first polarizer 12 is provided behind the lower glass plate (substrate) 13, and a second polarizer 18 is provided above the upper glass plate (substrate) 17. A volume reflection hologram (holographic light control film) 11 is provided behind the lower polarizer 12, to concentrate reflected light of a-single color into a specific viewing area (the prior art LCD shown in FIG. 2 is a monochromatic LCD).
In the prior art LCD shown In FIG. 2, the holographic film 11 is separated from the liquid crystal layer 15 by the lower glass substrate 13, and this will have a relatively large thickness. The spatial separation of the holographic film 11 and the liquid crystal layer 15 will introduce parallax into the display, and this will reduce the resolution of the LCD.
The prior art device described in U.S. Pat. No. 5,663,816 will also suffer from parallax problems.
U.S. Pat. No. 5,659,508 discloses an LCD viewable under ambient light comprising a liquid crystal panel, and a holographic reflective diffuser positioned behind the liquid crystal layer. The diffuser is made as a transmission hologram, and a light-reflective layer is deposited on the rear of the holographic diffuser. Such a holographic element is cheaper to manufacture as a broad band reflector than the other prior art holographic reflectors described above. However, the prior art device described in U.S. Pat. No. 5,659,508 again requires illumination with collimated light, and is not suitable for use with a closely positioned light source or with a high resolution color panel.
FIGS. 3A and 3B illustrate further prior art reflective LCDs. These are provided with polarization preserving optical diffusion films, so as to improve the performance and viewing angle of the display. Such polarization preserving films are manufactured by Microsharp.
The LCD shown in FIG. 3A comprises a glass substrate 20, a reflective pixel electrode 21, a liquid crystal layer 22, an indium tin oxide (ITO) electrode layer 23, a layer of color filters 24, a glass substrate 25. a Microsharp(trademark) transmission diffuser layer 26, a retardation film 27 and a polarizer 28.
The device shown in FIG. 3B is similar to that in FIG. 3A, but the Microsharp(trademark) diffuser 26 is omitted, and the reflective pixel electrode 21 of FIG. 3A is replaced by a transparent ITO electrode layer 21xe2x80x2. A polarizer 29 is further placed behind the lower glass substrate 20, and a metallized reflective diffuser 30 is placed behind the polarizer 29.
A transmission diffuser 26,30 is a relatively thick (100 xcexcm) plastic sheet with a random pattern distribution, selectable light diffusion profile, and a skew off-axis performance limited by 10xc2x0. The prior art devices illustrated in FIGS. 3A and 3B therefore require illumination with collimated light, and are not appropriate for use with a light source positioned close to the LCD. These devices have, moreover, a further disadvantage, since they produce a diffuse output, so that the contrast of the display will be low.
FIG. 4 illustrates a known projection LCD, which is described in C. Joubert et al., SPIE Proc. Vol.265, pages 243-252, 1996, and in C. Joubert et al., xe2x80x9cApplied Opticsxe2x80x9d, Vol.36, No.20, pages 4761-4771, 1997. The configuration of FIG. 4 includes a holographic microlens array for the LCD projection system.
The configuration of FIG. 4 has a full color liquid crystal display 31, which comprises red pixels 31R, green pixels 31G and blue pixels 31B. Light from a light source 32 is focused by a parabolic reflector 33 onto a holographic microlens array 34. Each individual element of the microlens array 34 is a transmission holographic lens which simultaneously disperses white light into RGB light components, and selectively focuses the dispersed RGB light components onto the corresponding color pixels (that is, the red component is focused on to a corresponding red pixel 31R, and so on). The holographic microlens array 34 is acting as both a lens and a diffraction grating.
The holographic microlens array 34 of FIG. 4 requires off-axis illumination by a well-collimated white light source having a divergence of less than 1.9xc2x0. Such an element is not suitable for illumination by a diverging light source that is positioned close to the LCD.
FIGS. 5A and 5B illustrate, respectively, a plane view and a top view of a prior art light source for illuminating a bar code. This prior art light source is described in U.S. Pat. No. 5,743,633. It includes a light emitting diode (LED) 35, a light shaping diffuser 36 and a lens 37. The light shaping diffuser 36 is a holographic surface diffuser, which produces a strip-shaped area of uniform illumination.
The illumination system of FIGS. 5A and 5B provides uniform illumination over a target area 38 of a bar code (i.e., a target uniformly illuminated area 38), by homogenizing and concentrating light from the LED 35.
WO 95/12826 discloses a reflective liquid crystal display device 500 as schematically shown in FIG. 5C, which includes a transmissive liquid crystal panel (transmissive SLM) 510 and a reflective holographic element (diffuser) 520 disposed behind the liquid crystal panel 510. When diffuse light or substantially collimated light is incident on the LCD 500, the holographic element 520 transforms the off-axis collimated beam into a collimated beam that is propagating substantially on-axis. Each point of the holographic element 520 of the LCD 500 of WO95/12826 has the same Bragg angle.
WO97/34174 and WO96/37805 both disclose a reflective display device provided with a holographic element. In both cases, the holographic element functions as a diffuser, and is intended to provide even illumination of the display and to increase the brightness of the display. These prior art documents are directed to eliminating the need to provide an auxiliary light source.
U.S. Pat. No. 5,594,560 discloses a reflective display device that is provided with a reflective holographic optical element. The holographic element is provided to enhance the brightness of the display, and the device incorporates a fluorescent layer in order to increase the spectral band of the hologram.
G. T. Valliath et al., xe2x80x9cDesign of Hologram for Brightness Enhancements in Color LCDsxe2x80x9d, SID DIGEST 98, pages 1138-1142 discloses a reflective display device comprising a transmissive holographic diffuser diffused in front of a reflective spatial light modulator. In use, the device is again illuminated with an off-axis beam of collimated light, and the holographic diffuser transforms this into a collimated beam propagating substantially on-axis. Each point of the holographic diffuser has the same Bragg angle.
The present invention provides a display device comprising: a spatial light modulator having an optical modulation layer; and a reflector disposed behind the optically modulating layer, and further comprising a holographic field lens.
A holographic field lens is a holographic optical element disposed substantially in the image plane of the display device, and which has optical power and re-direction properties Thus, a holographic field lens can convert an incident beam of diverging light to a beam of collimated light, and can also re-direct the incident light. Whereas the hologram layer 7 in the prior art device of FIG. 1A simply deflects the incident light, and so requires an incident beam of collimated light, the holographic field lens of the present invention simultaneously concentrates incident light and redirects it in a preferred direction, and may be used with a close, offset light source.
The present invention will function when the display device is illuminated with diverging light, as will be the case when it is illuminated by a nearby light source, but this will not be the case for the prior art device of FIG. 1A.
The reflector may be disposed within the spatial light modulator. This prevents problems from arising owing to parallax.
The reflector may be a non-Lambertian reflector. This increases the intensity of light reflected by the reflector.
The holographic field lens may collimate off-axis diverging light incident on the holographic field element. In this case, the display device functions as if it was illuminated with a virtual source of collimated light.
The collimated light may be directed at an angle xcex8 to the normal axis to the display, wherein xcex8 is greater than half the acceptance angle of the holographic field lens. This prevents light reflected back to the holographic field lens from interacting with the holographic field lens. The angle xcex8 may be xcex8xe2x89xa730xc2x0.
The holographic field lens may be a holographic microlens array. The display device may further comprise color filters, wherein the pitch of the holographic microlens array is substantially three times the pitch of the color filters. When such a device is illuminated with white light, the white light is split, with red light being directed to a red color filter, green light to a green color filter, and blue light to a blue color filter, respectively. Since the pitch of the microlens array is three times the pitch of the color filters, manufacturing tolerances for the microlens array are eased.
The holographic field lens may be disposed in front of the optical modulation layer. Alternatively, the holographic field lens and the reflector may be formed in a single element.
The holographic field lens may be disposed within the spatial light modulator, and the holographic field lens may be positioned close to the optical modulation layer. This reduces the loss of contrast that would be caused by hologram scatter. The holographic field lens may be separated from the optical modulation layer, to prevent holographic materials from contaminating the liquid crystal layer.
The display device may further comprise a light source, wherein the light source is arranged to illuminate the spatial light modulator with diverging light. This enables the display device to be operated in low ambient light.
The light source may be offset with respect to the spatial light modulator, so that specularly reflected light is not reflected towards a user.
The display device may further comprise a homogenizer disposed between the light source and the spatial light modulator. The homogenizer will shape the profile of the light beam from the light source to correspond with the shape of the spatial light modulator. It will also make the intensity uniform over the beam and thus over the display device. The homogenizer may be a holographic homogenizer. It may be a reflective holographic homogenizer, such as a reflective volume holographic homogenizer.
The light source may be substantially monochromatic, or it may be a white light source. Alternatively, the light source may comprise at least a first light source emitting light at a first wavelength and a second light source emitting light at a second wavelength. This allows the color balance of light from the light source to be adjusted, for example to suit the color filters used in a full color display device.
The light source may be a light emitting diode. Alternatively, the first light source may comprise a first light emitting diode for emitting light of the first wavelength, and the second light source may comprise a second light emitting diode for emitting light of the second wavelength. A light emitting diode is a low power device, and so is advantageous for use in a portable, battery powered display device.
The diffraction efficiency of the holographic field lens may vary over the holographic field lens. The diffraction efficiency of the holographic field lens may be lower at a side of the spatial light modulator near the light source and higher at a side of the spatial light modulator away from the light source. This improves the uniformity of the intensity of the display.
The holographic field lens may be chromatic, and the first and second light sources may be disposed at the respective foci of the holographic field lens for the light of the first wavelength and the light of the second wavelength, respectively.
The display device may further comprise a cover member, wherein the light source is disposed on the cover member. This is a simple way to provide a display device with a light source for use in low ambient light. Alternatively, the display device may further comprise a support member for supporting the light source, and the support member may be movable between a stowed position and a position in which the light source is able to illuminate the spatial light modulator.
The optical modulation layer may be a liquid crystal layer.
A second aspect of the present invention provides an illumination system comprising at least one light source, and a holographic homogenizer disposed in the path of light from the light source(s).
The holographic homogenizer may be a reflective holographic homogenizer.
The holographic homogenizer may alternatively be a transmissive holographic homogenizer. The illumination system may further comprise a reflector, wherein the transmissive holographic homogenizer is disposed between the at least one light source and the reflector.
The illumination system may comprise first, second and third light sources for emitting light at a first wavelength, a second wavelength and a third wavelength, respectively, wherein the first, second and third light sources are disposed in a linear array. The linear array of light sources is disposed at an achromatic angle to the holographic homogenizer.
The illumination system may comprise first, second and third light sources for emitting light of a first wavelength, a second wavelength and a third wavelength, respectively, wherein the light sources are arranged in a substantially triangular configuration.
Each of the at least one light source may be a light emitting diode.